Generally, a vehicle using an internal combustion engine using gasoline or heavy oil as main fuel has a substantial effect on production of pollutants such as air pollution. Therefore, to reduce the production of pollutants, automobile industries are developing an electric vehicle or a hybrid electric vehicle.
The electric vehicle is a vehicle which uses a battery engine operated by electric energy output from a battery. Since the electric vehicle uses a battery, in which a plurality of secondary batteries that may be charged and discharged are formed as a single pack, as a main power source, the electric vehicle produces no exhaust gas and produces minimal noise. Meanwhile, the hybrid electric vehicle, which is an intermediate stage vehicle between a vehicle using an internal combustion engine and an electric vehicle, is a vehicle that uses two or more power sources, for example, an internal combustion engine and a battery engine. Today, a hybrid electric vehicle of a mixed type using an internal combustion engine and a fuel cell directly obtaining electric energy by a chemical reaction of hydrogen and oxygen that is supplied continuously, using a battery and a fuel cell, or the like has been developed.
Meanwhile, since a vehicle that uses electric energy directly depends on performance of a battery, a state of the battery may be diagnosed by measuring a voltage, a current, a resistance, and the like of each battery cell. As a use time of the battery is increased, characteristics of the battery in which an electrochemical reaction is caused are changed by an irreversible reaction within the battery. As a representative change in characteristics, an internal resistance of the battery is increased due to a change in characteristics of the battery, and thus the reduction in voltage characteristic, temperature characteristic, capacity of the battery, and the like occurs. In other words, when the internal resistance is accurately measured, the battery state regarding the reduction of the battery characteristics may be diagnosed.
As the typical method for measuring an internal resistance of a battery, a method has been developed for connecting a separate apparatus to measure an internal resistance to a battery or a method for estimating an internal resistance by performing a modeling analysis on a voltage output with respect to an input current of a battery.
However, when the battery is mounted within a vehicle it may be difficult to separate the battery to measure an internal resistance of the battery and time, cost, and the like may be increased. Further, when a separate apparatus for measuring an internal resistance of the battery is mounted within the vehicle, part costs may increase. Further, the method for analyzing an output value with respect to an input using a modeling method may be difficult to apply to an embedded environment. Further, as a statistical method, a method has been developed for obtaining a resistance using a voltage-current distribution in which a current dispersion is equal to or greater than a constant value. Additionally, it may be difficult to define selection criteria for a threshold based on the current dispersion (e.g., standard deviation) and the current distribution.